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In the example below. the blue boxes represent each display that is connected. The white bar at the top of both boxes represents your menu bar and is shown on both displays when they are showing the same information.
When mirroring, your Mac attempts to match the closest resolution between the primary (or built-in) and secondary displays.
The usable resolution is the closest matching resolution for both the primary and secondary display. If you choose a resolution that is not supported on both displays, OS X reverts to using the closest matching resolution.
Extended desktop
With extended desktop, your displays are treated as one, continuous desktop.

Make sure each display is properly connected and powered on.

From the Apple (?) menu, choose System Preferences.

From the View menu, choose Displays.

Click the Arrangement tab.

Disable (uncheck) "Mirror Displays".

In the Displays pane of System Preferences, each blue box represents a display that is currently connected. The size of each box is relative to the current resolution of the display it represents. In the example pictured below, the larger box on the left represents an Apple 27" Thunderbolt Display, while the smaller box on the right is the built-in display of a 13" MacBook Air (Color LCD).
The white bar at the top of the left blue box represents the menu bar you see at the top of your display. Menu bar placement determines which display is treated as the primary display in OS X. To change your primary display, click and drag the white bar from one blue box to another.
You can also drag any of the blue boxes to change a display's represented location. Use this feature to make sure the connected displays match their real world location when navigating your desktop.
As shown below, each display retains its maximum resolution setting as they are operating as individual displays showing a continuous desktop.Press Command-F1 to toggle between video mirroring and extended desktop
Press Command-F2 to detect displays

To resolve the issue you can hold the power button of the monitor until
you get another message something similar to "OSD unlocked". Now if the
problem continues, switch off the monitor and remove the power cable then let it sit for a couple of minutes to 5 and
connect it again.

is the white square on the desktop or floating as a separate window? Can you use your PC and programmes OK, but it just looks rubbish?

If the latter I suspect that there is some sort of custom desktop which has been corrupted.

To change it right click anywhere on the desktop and choose properties. The choose desktop and swap the current one from anything on the list. Save and the background will change. If it is as I suspect the white square will also have vanished.

open the box, inside you will see up to 4 longish rows where your memory slots are. On these slots are white levers these levers are used to fit and remove the memory module. Make sure that you have earthed yourself agianst the metalwork of your computer or maybe a radiator or anything meatl before you attempt to remove the memory. Press the levers and the modules pop up. then you place the new memory inline with both ends ofthe levers and push fit.... dead easy no problem at all. Hey Presto memory fitted .... amen to that.